在水平地面上放一个竖直轻弹簧，弹簧上端与一个质量为2.0 kg的木块相连，若在木块上再作用一个竖直向下的力F，使木块缓慢向下移动0. 10 m，力F做功 2.5 J．此时木块再次处于平衡状态，力F的大小为50 N，如图所示，求：
（1）在木块下移0.10 m的过程中弹性势能的增加量．
（2）弹簧的劲度系数（g取10 N/kg）.
解：
（1）木块下移0.1 m的过程中，力F和重力做的功全部用于增加弹簧的弹性势能，故弹性势能的增加量为△Ep= WF+mgh= （2.5 +2.0×10×0.10）J =4.5 J.
（2）由平衡条件得，木块再次处于平衡时F=k·h．所以劲度系数
Recently I overheard a father and daughter in their last moments together at the airport. The airline had her departure and standing near the security gate, they hugged and he said, “I love you. I wish you enough.” She said, “Dad, our life together has been more than enough. Your love is all I ever needed. I wish you enough, too, Dad.” They kissed and she left. He walked over toward the window where I was seated. Standing there I could see he wanted and needed to cry. I tried not to him, but he welcomed me in by , “Did you ever say good-bye to someone knowing it would be forever?” “Yes, I have,” I replied. “Forgive me for asking, but why is this a forever good-bye?” “I am old and she lives much too far away. I have challenges ahead, and the reality is, the next trip back will be for my ___ ” he said. “When you were saying good-bye I heard you say, ‘I wish you enough’. May I ask what that means?” He began to smile. “That’s a wish that has been for many generations within my family. My parents used to say it to everyone.” He for a moment, looking up as if trying to remember it in detail, and then he smiled and the following: “I wish you enough sun to keep your attitude bright. I wish you enough rain to appreciate the sun more. I wish you enough happiness to keep your spirit alive. I wish you enough pain so that the joys in life appear much bigger. I wish you enough gain to satisfy your wanting. I wish you enough loss to all that you possess. I wish you enough ‘Hellos’ to get you enough the final ‘Good-byes’.” Then he walked away, leaving me lost in thought.
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